1. Field of the Invention
The present invention is directed to a cassette for use in a volumetric infusion pump which is particularly adaptable for facilitating administration of fluids to a patient. More specifically, the present invention is related to a cassette which employs a diaphragm in order to seal the interior of the cassette from the outside environment and to facilitate operation of the cassette.
2. Background of the Invention
Various methods have been developed for the administration of fluids to a patient. This is particularly true with regard to intravenous administration of fluids. The most common method of administering such fluids is through the use of a standard intravenous ("IV") administration set. The use of such an intravenous administration set generally requires suspending a container of fluid at an elevated position with respect to the patient. The container is then placed in communication with the patient by way of a series of tubes and connections as needed and desired, and finally through a needle disposed within the patient's vein. The administration of fluid occurs by the operation of the hydrostatic head created by the elevated container which is communicated through the infusion line to the patient.
When administering fluids intravenously to a patient by any chosen method, it is important that the rate of administration and the total quantity of fluids administered be accurately controlled. In the case of the IV administration set, the fluid from the elevated container must generally pass through a drip chamber where the number of drops per minute provides an approximate estimate of the rate of flow of fluid to the patient. The medical personnel in charge of the administration must, therefore, frequently check the drip rate and the amount of fluid left in the bottle in order to provide the necessary control. When it is necessary to adjust the rate of flow, the drip rate must be manually adjusted. Likewise, in order to stop the administration of fluid when the proper quantity of fluid has been administered, it is necessary to manually terminate the flow. The frequent manual intervention by medical personnel in the process of intravenous fluid administration makes the process time consuming, costly, and not particularly accurate.
Another serious problem with conventional IV administration sets relates to introducing foreign substances to the patient. Because of the need for frequent manual supervision of the process, there is always the possibility that air will enter the system and subsequently be introduced to the patient's blood stream. The introduction of air through a patient's vein can have devastating results. Similarly, the frequent handling of the components of the IV set, along with the related need to replace tubing, fluid containers and the like, increases the possibility of infection.
In order to overcome some of the problems encountered with the standard IV administration set, numerous pump devices have been proposed and used in the art for pumping fluids to a patient. Some of these pump devices, for example, use electronics means to detect the drip rate in a drip chamber so that the rate of flow can automatically be adjusted.
Certain types of pumps may employ a chamber member to apply pressure to an IV tube and, thus, provide a positive pumping action to the fluid being administered to the patient. While in some settings it is found that this type of pump is desirable, its use is still limited in that the rate of pumping and volume pumped are not easily controlled.
Another type of pump which has been used eliminates the elevated bottle of fluid and uses in its place a large volumetric chamber. Fluid is pumped from the large volumetric chamber by slowly introducing a large piston into the chamber and correspondingly forcing fluid from the chamber. The rate of flow from the chamber is controlled so that the volume of fluid introduced to the patient and the flow rate are maintained within acceptable ranges. This type of pump also has several disadvantages. Such pumps are found to be costly and difficult to maintain. Further, if the rate of infusion is low, it is difficult to accurately control the rate of infusion from the large volumetric chamber.
An additional type of pump introduces fluid to a patient from a small volumetric chamber. Typically, a piston moves reciprocally within the small volumetric chamber drawing fluid into the chamber as the piston withdraws from the chamber and then forcing fluid out of the chamber as the piston re-enters the chamber. One such pump is disclosed in U.S. Pat. No. 3,985,133 to Jenkins, et al. (Oct. 12, 1976). In that type of volumetric infusion pump, fluid from a conventional container flows into the device through an inlet line. In order to facilitate the flow of fluid into the device, the fluid container is generally elevated with respect to the pump. The fluid is pumped as described above and then leaves the pump through an outlet line for introduction to a patient.
In this type of pump, a removable "cassette" may be inserted into the pump during operation. The cassette includes a chamber of a predetermined volume, as well as a plunger piston which reciprocates within the chamber. The cassette operates on the same general principle as a conventional syringe with the plunger piston pulling fluid into the chamber as it is withdrawn from the chamber, and then subsequently forcing fluid back out of the chamber. The pump generally includes a drive means which is coupled to the plunger to control the pumping action so that fluid will enter and leave the cassette at a predetermined rate. One type of drive means is a stepping motor which is set at a controlled rate in accordance with the number of pulses required to pump the desired volume.
The volume of the chamber in the cassette is relatively small compared with the volume of fluid which is normally infused. This provides for precise control of the amount of fluid infused and the rate of infusion. At the same time the small size allows the cassette to be manufactured and sold relatively inexpensively. Such a cassette can, for example, be presterilized and disposable.
Using such an apparatus, the pump itself never comes in contact with fluid infused, so that the pump does not need to be sterilized after each use. The pump can be used repeatedly and only the cassette needs to be replaced. Further, it is only necessary to be concerned about the sterile condition of the cassette portion of the apparatus and the associated inlet and outlet lines.
In use, the cassette is supported on the pump so that a drive means of the pump engages an extension of the cassette plunger. Tubing is connected from the fluid reservoir to an inlet corridor of the cassette so that fluid may be introduced into the interior of the cassette as the plunger withdraws from the interior of the volumetric chamber. As was mentioned above, once the chamber is filled with the desired volume of fluid, the direction of the plunger reverses and re-enters the volumetric chamber, forcing fluid out an outlet corridor. The direction of flow of fluid into and out of the cassette is controlled by a two-way valve so that when the plunger is withdrawing from the interior of the volumetric chamber, fluid flows into the chamber, and while the plunger again proceeds into the volumetric chamber fluid is directed out the outlet corridor and toward the patient.
While this device has many desirable features, several problems remain. One of the primary problems relates to the means of sealing between the cassette body and the plunger. It will be appreciated that an effective seal is required in order to prevent fluid leakage from the volumetric chamber. Various means of sealing have been attempted but have been found to be less than satisfactory. Such means include the providing of an O-ring around the base of the cassette body at the intersection of the plunger. Likewise, it may be possible to simply provide close fitting engagement between the plunger and the cassette body (together with a flexible material along part of the plunger) such that sealing engagement may be obtained. It will be appreciated, however, that even using these means, leakage may be a problem. The plunger must reciprocate rapidly and continuously within the cassette body. As a result, fatigue and wear are often encountered which may result in failure of the seal.
A further problem encountered in the art is that of contamination of the interior of the volumetric chamber. Since the fluid flowing through the volumetric chamber will likely be introduced directly into the bloodstream of a patient, it is necessary to scrupulously maintain a sterile environment. It is particularly important to provide a bacterial seal, as well as a liquid seal, between the exterior environment and the interior of the volumetric chamber. The sealing means used to date for sealing the plunger and the volumetric chamber have been inadequate in providing such a bacterial seal. Simple O-rings or friction engagement will likely allow bacteria to enter the chamber by traveling in what is essentially a straight line. No effective barrier is provided.
From the above discussion it is apparent that what is currently needed in the art are improved means for sealing the volumetric cassette from the outside environment. It would be an advancement in the art to provide such sealing means which provided an improved liquid seal and at the same time, was able to withstand the repeated reciprocating movement of the plunger within the volumetric chamber. It would also be an advancement in the art to provide such sealing means which was extremely low cost so that the sealing means, along with the entire cassette, could be presterilized and disposable if desired. It would be a further advancement in the art to provide such sealing means which provided an effective block against bacterial intrusion into the interior of the volumetric chamber.
Such methods and apparatus are disclosed and claimed below.